Diaphragm carburetors are primarily used in handheld devices, e.g. power saws, or in model airplanes, which respectively have to function independently of their position. Known control diaphragms are made of fabric-reinforced rubber (e.g. DE202005020877) and centrally feature a circular, rivet-fastened reinforcing plate in a central sensing region (e.g. EP0608490). An example of such a control diaphragm is illustrated in FIGS. 1-3. The control diaphragm is held in the carburetor housing with a peripheral fastening border 3 and closes a control chamber 5 in a sealing manner. A spring-loaded controlling lever 4 is arranged in the control chamber and presses against the central sensing region 1 of the control diaphragm in the region of the reinforcing plate 6 in order to thereby sense a diaphragm stroke or an axial deflection of the sensing region 1, respectively. The control chamber 5 is connected to a carburetor chamber of the diaphragm carburetor. The side of the control diaphragm facing away from the control chamber is subjected to the atmospheric pressure. During the operation of the carburetor, a vacuum in the range of a few millibar being generated in the carburetor chamber or in the control chamber causes a diaphragm stroke of the central sensing region 1 in the tenth of a millimeter to millimeter range, wherein said diaphragm stroke is sensed by the controlling lever 4 and used for controlling the fuel feed. In order to increase the maximum diaphragm stroke, the control diaphragm features a peripheral corrugation 8 that concentrically extends around the reinforcing plate 6.
The rivet-fastened reinforcing plate, which has a radius of more than 50% of the radius up to the peripheral fastening border 3, reinforces an extensive inner region of the control diaphragm. The reinforcing plate causes a uniform diaphragm stroke over the region covered by the reinforcing plate (see arrows in FIG. 3). In other words, this relatively large central sensing region ideally oscillates uniformly in the axial direction. Under realistic conditions, however, the reinforcing plate 6 tends to respectively “flutter” or wobble, i.e. the reinforcing plate 6 can be easily tilted out of the diaphragm plane, particularly during fast position changes of the carburetor, and thereby lead to irregularities in the carburetor control during its operation. These position-dependent irregularities are also intensified by the mass of the reinforcing plate 6.
Another problem of known control diaphragms can be seen in that the rubber coating comes in contact with fuel or fuel vapors during the operation of the engine. This leads to swelling of the rubber layer. While the engine is at standstill, the rubber layer dries and its swelling decreases again. Both processes take place randomly, but also affect the response and control behavior of the control diaphragm. In addition, frequent swelling and drying leads to an increased formation of cracks in the rubber layer.
A control diaphragm of this type is also manufactured in a multi-stage process such that it is relatively common for residual tensions, e.g. of the processed textile and rubber materials, to persist in the control diaphragm to different degrees and in random distribution. These residual tensions uncontrollably affect the control behavior. In addition, a control diaphragm with rivet-fastened reinforcing element is susceptible to leaks and cracking.
In order to partially eliminate the above-described problems, DE3827555 proposes a one-piece control diaphragm of polytetrafluoroethylene (PTFE). Instead of the rivet-fastened reinforcing plate, this control diaphragm features a reinforcing part that is either directly formed on the control diaphragm or connected to the control diaphragm by means of welding or bonding. Due to the mass concentration caused by the respective reinforcing plate or reinforcing part, however, the “fluttering” problem and the problem of position-dependent irregularities are not solved with this control diaphragm. This solution has not been able to establish itself on the market.
Another control diaphragm is known from WO2014018723. This control diaphragm has a multilayer structure with a continuous layer and an interrupted layer. The continuous layer closes the control chamber in a sealing manner and reacts to the pressure changes in the control chamber. The overall flexibility of the multilayer control diaphragm is highly dependent on the shape of the interrupted layer, which is realized, for example, in the form of a flat coil spring. The control diaphragm features a reinforcing body in the central sensing region. The manufacture of this control diaphragm is also relatively elaborate. In addition, possible deposits between the two layers can lead to irregularities during the operation of the carburetor.
Measurements on a known control diaphragm consisting of rubber-coated fabric and a rivet-fastened reinforcing plate show a pressure/displacement characteristic (i.e. a deflection of the sensing region in dependence on the pressure difference in the control chamber), which significantly flattens starting at a vacuum of approximately 4 millibar (see FIG. 7(a)). In other words, the control sensitivity is significantly reduced at operational vacuums between 4 and 8 millibar.